Document that reader is implemented as a consumer (#5556)
Document that reader is implemented as a consumer (#5556)

### Motivation

I would have found it helpful to know earlier that a reader is implemented in terms of a consumer.

merlimat/pulsar,massakam/pulsar,massakam/pulsar,massakam/pulsar,yahoo/pulsar,massakam/pulsar,yahoo/pulsar,massakam/pulsar,yahoo/pulsar,yahoo/pulsar,merlimat/pulsar,yahoo/pulsar,merlimat/pulsar,merlimat/pulsar,merlimat/pulsar,massakam/pulsar,yahoo/pulsar,merlimat/pulsar